A metal detector of this type operating in accord with the Pulse Induction process (PI) Mode is known from DE 103 01 951 A9. The interaction between the primary and secondary coils is decoupled by partially overlapping the co-planar coil systems. The decoupling is adjusted by means of mechanically displaceable masses in the region of the overlap or by means of auxiliary electrical compensating means e.g. in the form of additional compensation signals from the generator in the receiving circuit. They compensate for the incompletely decoupled component of the transmitting power in the receiving coil. A “feedback” between the detected signal in the receiving coil and the compensation effect, i.e. a closed regulating system is not present.
A comparable arrangement is known from DE 103 18 350 B3, wherein a plurality of coils are nested in overlapping, neighbouring but mutually displaced manner in regard to their magnetic alternating field. The largest coil, preferably the receiving coil determines the periphery of the coil arrangement.
A reversal of the principle specified above i.e. a surrounding transmitting coil with two receiving coils in the form of an “eight” is known from DE 36 19 308 C1, in which the transmitted field is mutually cancelled.
A metal detector comprising a transmitting coil and a receiving coil is known from DE 43 39 419 C2, these coils partially overlapping each other in such a way that the alternating inductance factor is minimal. The coils are operated alternately as transmitting and receiving coils.
In order to reduce capacitive cross-talk between the transmitting coil and the receiving coil when the transmitting and the receiving coil are located very close together as is the case for printed coils, a screening means in the form of a screening electrode between the transmitting and the receiving coil is proposed in DE 10 2004 047 189 A1. Further auxiliary windings are provided for fine adjustment purposes.
An amplitude regulating system wherein light signals are detected and compensation is made for external interference such as stray light and temperature or aging effects between light emitters and light receivers is known from EP 706 648 B1. The light emitters are operated periodically and alternately by a clock pulse generator. The light that has been amplitude regulated in at least one light path reacts as necessary in the light receiver with the light from a further light emitter such as a compensating light source for example in such a way that a received signal without clock synchronous signal components is produced. The signal received by the light receiver is supplied to a synchronous demodulator which breaks down the received signal into the signal components corresponding to the two sources of light. These are compared with one another in a comparator, whereby a state of the control value corresponding to a zero state is developed. If a signal corresponding to this zero state is not present at the output of the comparator, then this control value is used in order to appropriately regulate the radiating power which is being supplied to the sources of light until this state is reached.